The present invention relates generally to a window structure for transmitting x-ray radiation and for shielding undesirable debris resulting from the x-ray radiation generation process.
A variety of window systems have been developed for irradiating samples. By way of example, Forsyth et al. in U.S. Pat. Nos. 4,980,896 and 4,697,934; Riordan et al. in U.S. Pat. No. 4,837,794 and Grobman in U.S. Pat. No. 4,408,338 each describe a method of x-ray lithography of semiconductor chips. In fact, the use of x-ray lithography is often times preferred because of its ability to produce line widths less than one micron. Soft x-rays (i.e. relatively long wavelengths and low penetrating power) are particularly useful for such applications. Soft x-rays can be generated by a variety of known techniques; however, such x-ray generation processes can also produce unwanted debris which can adversely interfere with the x-ray lithography process. In one x-ray lithography system, a pulsed plasma source is used for x-ray generation. Such sources convert an electrical input into x-rays using the phenomena of gas jet z-pinch. In this method of x-ray generation, a burst of a gas (e.g. nitrogen, krypton, or argon) is expanded using a nozzle in concert with the fast discharge of a capacitor bank through the expanding gas. A high current discharge and the resulting intense magnetic field radically compresses the plasma. The result is a dense, high temperature plasma which is a very intense source of desirable x-rays with comparatively long wavelengths and hence, low penetrating power (i.e. soft x-rays). Unfortunately, generated along with the x-rays are hot gases, charged particles and other debris having instantaneous accelerations exceeding 100 g's.
Consequently, a need exists for a window structure which allows transmission of the x-rays, yet blocks or shields the sample from undesirable radiation generated debris. For electromagnetic radiation above about 1000 .ANG. in wavelength, or below about 1 .ANG. in wavelength, practical transmissive debris shield materials exist, (e.g. quartz and beryllium). However, for electromagnetic radiation between about 1000 and 1 .ANG. in wavelength, no single practical window material exists. Known durable window materials are not sufficiently transparent to electromagnetic radiation within this range while window materials which are sufficiently transparent within of this range are not very durable. Unfortunately, this is precisely the range in which high resolution microcircuit lithography is contemplated. Satisfying these dual, competing requirements has been greatly impeded because no one material or structure has been discovered which exhibits both the required transmissivity for x-rays and the structural strength to withstand the impact of debris. As such typical x-ray lithography systems employ a first structure as a window and a second, spaced apart structure as a debris shield. See e.g. Riordan et al., Grobman. More recently, Perkins et al. in U.S. Pat. Nos. 4,960,486 and 4,933,557 have proposed a structure composed of an x-ray transmissive film material overlaid onto a structural support.
In spite of such advances, a need still exists for a single window structure combining both transmissive and debris shielding capabilities. The present invention provides a novel x-ray transmissive shield composed of materials having complementary properties so as to overcome the limitations of existing window and debris shield systems.